Programmed cell death 1 ligand 1 (PD-L1) is a member of the B7 family of immunological modulating molecules that has been demonstrated to have an imnumoinhibitory function mediated through interactions with the PD-1 receptor, as well as to have costimulatory function in some contexts through interactions with an as yet unidentified receptor (U.S. Pat. No. 6,936,704; U.S. Pat. Publ. 2009/0317368; Keir et al. (2008) Annu. Rev. Immunol. 26:677-704; and Xu et. al. (2013) PLoS One 8:e56539). PD-1 is a member of the immunoglobulin family of molecules (Ishida et al. (1992) EMBO. J. 11:3887; Shinohara et al. (1994) (Genomics 23:704) and is believed to play a role in lymphocyte survival, e.g., during clonal selection (Honjo (1992) Science 258:591; Agata et al. (1996) Int. Immunology. 8:765; Nishimura et al. (1996) Int. Immunology 8:773) based on its function as an inhibitory receptor similar to that of CTLA4 (Wu et al. (2012) Int. J. Biol. Sci. 8:1420-1430). While engagement of a costimulatory receptor results in a costimulatory signal in an immune cell, engagement of an inhibitory receptor, e.g., CTLA-4 or PD-1 (for example by crosslinking or by aggregation), leads to the transmission of an inhibitory signal in an immune cell, resulting in downmodulation of immune cell responses and/or in immune cell anergy. While transmission of an inhibitory signal leads to downmodulation in immune cell responses (and a resulting downmodulation in the overall immune response), the prevention of an inhibitory signal in cells, such as immune cells, leads to upmodulation of immune cell responses (and a resulting upmodulation of an immune response).
Numerous blocking antibodies targeting PD-L1 are currently under review in clinical trials for treating a number of immune-related disorders (reviewed in Keir et al. (2008) Annu. Rev. Immunol. 26:677-704; Wu et al. (2012) Int. J. Biol. Sci. 8:1420-1430; Sakthivel et al. (2012) Rev. Recent Clin. Trials 7:10-23; Flies et al. (2011) Yale J. Biol. Med. 84:409-421; Topalian et al. (2012) Curr. Opin. Immunol. 24:207-212; Sarasella et al. (2012) Curr. Mol. Med. 12:259-267; Riella et al. (2012) Am. J. Transplant. 12:2575-2587; and Inozume (2013) Nihon Rinsho Meneki Gakkai Kasishi (2013) 36:134-138). However, the anti-PD-L1 antibodies used in such trials have several disadvantages. First, they recognize and bind to the extracellular domain of PD-L1. While recognizing such epitopes disrupt interactions with PD-L1 receptors, such epitopes do not allow the antibody to distinguish between membrane-bound forms of PD-L1 versus soluble forms of PD-L1. Soluble forms of PD-L1 have been determined to have distinct structural characteristics and biologically relevant functions relative to membrane-bound forms of PD-L1 (U.S. Pat. No. 6,936,704; Chen et al. (2011) Cytokine 56:231-238; Frigola et al. (2011) Clin. Cancer Res. 17:1915-1923; and Frigola et al. (2012) Immunol. Lett. 142-78-82). For example, concomitant recognition of soluble PD-L1 presents high and undesired background staining upon immunohistochemical analyses of membrane-bound PD-L1 protein. Second, anti-PD-L1 antibodies targeting the extracellular domain of PD-L1, which represents the vast majority of surface availability for antibody recognition, will bind to and sequester the protein when administered for therapeutic or other uses, such that additional areas of protein recognition useful for continued monitoring, diagnosis, and prognosis of PD-L1 expression and activity will be hindered.
Accordingly, there is a need in the art to identify new anti-PD-L1 antibodies having a specificity and sensitivity for cytoplasmic portions of membrane-bound PD-L1.